Alien X appears in Ben 10: Alien Force and Ben 10: Ultimate Alien. Ben is able to transform into Alien X using the Omnitrix.

Alien X is on of Ben’s his most powerful aliens; however, he cannot afford the cost that comes with becoming Alien X.  Alien X is able to warp reality at a whim. Alien X is also made up of 2 differing personalities (3 including Ben). Serena is the voice of love and compassion and Bellicus is  the voice of rage and aggression. In order for Ben to complete any tasks he needs to convince Serena and Bellicus to agree to go along with it.